1. Field of the Invention
This invention relates to an alignment layer for a liquid crystal display device and a liquid crystal display device provided with this alignment layer. More particularly, it relates to an alignment layer, by which an alignment state capable of giving a wide viewing angle can be controlled, and a liquid crystal display device wherein this alignment layer is used.
2. Description of the Related Art
A liquid crystal display device is an electro-optical device containing a liquid crystal material which undergoes changes in optical characteristics when an electrical field is externally applied. In recent years, liquid crystal display devices of the twisted nematic (TN) type have been widely applied to television sets, personal computers, etc., since they are thin and light weight and consume less electricity.
In a liquid crystal display device of the TN type, a liquid crystal alignment layer is formed on a substrate made of, for example, glass and the surface of the alignment layer is rubbed with a cloth, etc. so as to align liquid crystals, i.e., a so-called rubbing treatment. Namely, a liquid crystal alignment layer is obtained by forming a thin film of a heat-resistant polymer (for example, polyimide, polyamide) by spin coating, printing, dipping, etc. and drying or setting the polymer film followed by rubbing. In such a liquid crystal alignment layer, liquid crystal molecules are aligned in one direction while defining a pretilt angle of usually 2 to 4.degree. from the surface of the substrate. Also, attempts have been made to produce alignment layers by obliquely depositing an organic substance (silicon oxide, etc.) onto a substrate (oblique deposition).
As a basic drawback, however, the characteristics of a liquid crystal display device are highly dependent on the viewing angle. That is to say, it suffers from serious problems including change of the contrast depending upon the viewing angle and black/white inversion of the display.
To overcome this problem in the viewing angle dependency, there have been recently proposed a liquid crystal display device provided with two regions, which differ from each other by 180.degree. in the upstanding direction of liquid crystal molecule, in one image element (K. H. Yang, IDRC, p. 68, 1991, hereby incorporated by reference) and a liquid crystal display device provided with two regions, which differ from each other in pretilt angle, in one image element (Y. Koike, et al., SID, p. 798, 1992, hereby incorporated by reference). In these methods, however, two regions differing in alignment state should be formed in each of fine image elements, which requires photolithographic techniques, etc. Accordingly, such a method involves an elevated number of steps and a complicated process. It is therefore highly difficult to economically produce a liquid crystal display device at a high efficiency thereby. Although there has been proposed amorphous TN alignment which requires no rubbing process (Y. Toko, S. Kobayashi, et al., SID, p. 622, 1993, hereby incorporated by reference), this method suffers from a problem that the whole display has a deteriorated contrast.
Accordingly, it has been urgently required to develop a method for improving the viewing angle dependency by using an alignment layer material, thus solving these problems encountered in the prior art and providing a liquid crystal display device with a less viewing angle dependency.